The present invention relates to an inkjet recording head with a simple structure that can solve the problems caused by a decrease of ink viscosity by dissipating the drive heat generated during ink ejection, which occurs during high speed drive of the recording head. (PRIOR ART)
Conventionally, a so-called shear mode type inkjet recording head (hereinafter, referred as a shear-mode head) is known in the industry, in which an actuator substrate is structured with a plurality of grooves on a polarized piezoelectric element, a plurality of pressure generation rooms partitioned by said piezoelectric element are formed by adhering a cover plate onto the upper surface of said actuator substrate, said piezoelectric element is deformed by applying an electric voltage between the adjacent pressure generation rooms, and ink is ejected from nozzle holes provided in a nozzle plate.
In this shear-mode head, since ink channels to be filled with ink are formed in the piezoelectric element, when the piezoelectric element generates heat, this heat is transferred to and heats the ink. If the temperature of the ink is raised by heating, the viscosity of the ink is reduced and ink ejection speed increases and the landing position of the ink tends to deviate from the targeted position to cause significant degradation of image quality.
For this reason, in the shear-mode head, without employing a positive heat dissipating measure, or with an insufficient heat dissipating measure, the heat generated in the piezoelectric element has no root of dissipation, and since the heat is transferred to the ink, the viscosity of the ink decreases, the ejection speed of ink drops increases and this causes landing position errors with regard to the recording medium moving at constant rate, and resulting in degradation of image quality.
This heat generation phenomenon affects sensitively the image quality. For example, when ink drops of 20 pl are continually ejected for 2 sec by the drive voltage of 20 V, at a frequency of 17 kHz—this condition can be expressed more specifically based on an actual case—when an ink head carriage moves and prints reciprocally between the edges of a 1350 mm wide width recording medium, with the rate of 600 mm/sec, for the printing of one onward way or homeward way it takes about 2 sec., in this condition, the ink ejection speed at the end of 2 sec. of ejection increases more than 0.1 m/sec compared to the start of the ejection, and the printed image density varies more than 0.01. This is due to heat generation of the head, heat transfer to the ink and decrease of the ink viscosity.
During the time when ink ejection stops for switching the ink carriage moving direction from onward to home ward, the temperature of the head decreases, and after switching the direction, printing is again conducted resulting in variation of image density of more than 0.01 between the start and end of printing. A density difference of 0.01 seems to be a rather small value, however, this density difference appears at adjacent positions in the right and left end of the wide-width recording medium, and this density variation can be visible to the naked eye. In order to make this density variation invisible to the naked eye, it is necessary to reduce the ink ejection speed raise due to heat generation of the head to less than 0.1 m/sec., and to reduce the density variation to less than 0.01, during 2 sec. of continuous ink ejection.
The raising rate of the ink ejection speed increase with respect to the ink temperature increase varies between low viscosity ink and high viscosity ink. For example, as for the high viscosity ink having the viscosity of 10 cp at room temperature, it is known that the ink ejecting speed increases 0.3 m/sec. for every 1° C. temperature raise. Therefore, in cases where a wide-width recording medium is printed from edge to edge of the width, the ink temperature increase in the head is necessary to be restricted within 0.3° C. If ink ejection is continued for a long time, the heat accumulated in the piezoelectric element is transferred to the ink, and the ink temperature gradually increases. Usually, a head is structured such that a thermistor is provided on the head which detects the ink temperature to control the drive voltage of the head to keep the ink ejecting speed constant, however, there are about 10 seconds delay for its response, and this can not adequately respond to the temperature increase which occurs during one line of printing with not more than 10 sec.
Although, temperature increase of the head by the continuous ink ejection during one line printing with about 2 seconds for wide-width recording medium is expected to be rather small, the preferable countermeasure for preventing the temperature increase during such a short time is to improve dissipation of the head to prevent the heat generated in the piezoelectric element from flowing toward the ink.
In the prior art, the technologies are known in which a drive circuit IC is incorporated inside a high heat-conductive and electrically insulative ceramic board to dissipate the heat generated by the IC (refer to patent article 1), and other technology in which a heat generating element is adhered on to a high heat-conductive board by a high heat-conductive film adhesion tape (refer to patent article 2) are known.
However, only a countermeasure such that the member contacting the piezoelectric element is constituted with high heat-conductive material for dissipating the heat of the piezoelectric element is not sufficient to overcome the following problems.
Namely, since in the shear-mode head, ink ejection amount and ink ejection frequency are largely determined by the length of the ink channel, in order to eject sufficiently small ink drops at high frequency, it is necessary to make the length of ink channel not more than 5 mm. For this reason, a piezoelectric member with a length of several cm is necessary to be cut out to make prescribed length of the members, after the member is ground to form grooves, electrodes are formed, and a cover plate is adhered onto the top surface of the formed ink channels. In cases where physical properties of the piezoelectric element and that of the cover plate are greatly different with each other, for example, in the case of ceramics material harder than the piezoelectric element being used for the cover plate, if grinding conditions are set based on the harder ceramics material, the piezoelectric element, which is a less hard material, can be excessively ground resulting in excessively large grooves for ink channels. On the contrary, if the grinding conditions are set based on the piezoelectric element of less hard material, the harder cover plate material cannot be cut well enough. Since a nozzle plate is adhered on this cut surface, forming a nozzle for ejecting ink, if the cut surface is rugged the nozzle plate cannot keep a flat surface and this results in the problem of deflecting the ink ejection direction from the nozzle.
For this reason it is required to adhere the nozzle plate only after the cut surface of the piezoelectric element is polished and smoothed. The polishing requires a considerably long time and is a troublesome process, and further, can lead to problems of clogging and contamination in the ink flow path during the process.
For the piezoelectric element, PZT is frequently used. Since PZT has a Young's modulus of about 50 Gpa, which is a rather small value for ceramics, if it is ground with a diamond cutter, a smooth ground cut surface can be obtained. Other popular ceramics, alumina for example, has a high Young's modulus of about 300-400 Gpa, therefore, the member obtained by adhering the PZT and the alumina is difficult to cut by grinding, and a smooth cut surface cannot be obtained, which requires an additional time consuming process to polish the cut surface.
As for the cover plate, using the same material as the piezoelectric element is preferable from the viewpoints that thermal expansion does not need to be considered and a smooth cut surface can be obtained. However, the piezoelectric element, for example, consisting of PZT has a low thermal conductivity of 1.5-2.0 W/mK, and the heat generated inside the piezoelectric element is hard to dissipate. Namely, if the same material as the piezoelectric element is used for the cover plate, the ink channels are enclosed with materials of low thermal conductivity, so the heat generated in the piezoelectric element is hard to dissipate, which eventually leads to the increase of ink temperature.
Further it is known to use a ceramics with high thermal conductivity as the cover plate covering the upper surface of the ink channels made on the piezoelectric element, and to adhere them with an adhesive with high thermal conductivity (refer to patent article 3), however, said problems regarding the grinding process are not mentioned in the prior art.                Patent article 1: TOKKAI-HEI NO. 10-217454        patent article 2: TOKKAI NO. 2001-150680        Patent article 3: TOKKAI NO. 2000-135788        
The objective of the present invention is to improve heat dissipation from the piezoelectric element by using a material with higher thermal conductivity than the piezoelectric element as the cover plate, and to prevent deformation or separation of the inkjet head which used to occur during the use of the head or in the manufacturing process, by using materials having a similar thermal expansion coefficient with the piezoelectric element, and further to provide an inkjet head of high reliability, which does not require in its manufacturing process to polish the ground cut surface of the member constituted by adhesion of the piezoelectric element and the cover plate.